If it drives you crazy that Outlook seems to randomly double-space your double-spaced e-mails, making them quadruple-spaced, Slipstick.com has the best explanation of the cause:
When you press Enter while creating HTML email, Outlook inserts a paragraph tag (<p>), so two Enters (for white space between the paragraphs) inserts two <p> tags, which is double spaced when rendered in a browser. If you use Shift+Enter twice, which creates the line break tag (<br>), the message will not look double spaced in a browser. This will look ok in all mail clients.
… and lists the six best ways to stop Outlook from double-spacing your e-mails:
- Use plain text for messages. Plain text will look ok in any and every email client available.
- Press Enter once, not twice, when composing HTML formatted email.
- Press Shift+Enter twice at the end of a paragraph, rather than pressing Enter twice.
- Just ignore it as an inconstancy in the way various applications handle HTML.
- Edit the email template to add ’12 points after’ in the Normal style.
- Use Search and Replace to replace paragraph symbols (^p) with a manual line break(^l).